Optical fiber amplifiers and lasers have rapidly become important components of optical communications systems. Optical fiber amplifiers are used to intensify optical signals that are attenuated along an optical fiber communication path. They have replaced cumbersome electrical repeaters in fiber-optic communication links allowing true all-fiber optical communication systems to be realized. Similarly, optical fiber lasers have been proposed to generate an optical carrier for fiber-optical communications systems. These lasers can be externally modulated to carry digital or analog information and, in some cases, are an alternative to laser diodes as sources of high-power light in optical fiber communication systems.
Both fiber lasers and amplifiers operate on similar principles. The silica glass in the guided-wave region of the optical fiber is doped with traces of a rare-earth element such as erbium or praseodymium, which exist in the triply ionized state in a glass matrix. The energy structure of these ions is such that an optical signal of a particular wavelength propagating in the guided-mode region of the fiber can be amplified if the ions are pumped into an excited energy level by the addition of, for example, electromagnetic radiation of the appropriate energy. In the case of erbium, for example, pumping the ions with light of wavelength 980 nm or 1480 nm will configure the erbium ions to amplify signal light within a wavelength range of approximately 1535 nm to 1565 nm. In the case of praseodymium, pumping the ions with light of wavelength 1017 nm will configure the praseodymium ions to amplify signal light within a wavelength range approximately 1300 nm to 1320 nm. This is the basis for optical fiber amplifiers. If a further mechanism is established to recirculate this amplified signal in the fiber by placing appropriate reflectors at the ends of the fiber, then laser action can occur if the net gain in the fiber equals the loss within some optical bandwidth. Most modern optical fiber communication systems utilize laser sources at approximately 1300 nm or 1550 nm, so fiber amplifiers that operate at a either of these wavelengths are of considerable importance.
The excitation or pumping of rare earth doped optical fibers is currently accomplished most conveniently by light from semiconductor laser diode sources that is directed into the end of the doped fiber by means of focusing system such as one comprised of lenses. Current focusing systems couple between 40% to 90% of the light from such laser diode sources into the fiber; commercially available systems typically having coupling efficiencies of 50% to 60% for laser diodes that emit light in a single spatial mode. The guided mode region of the fiber is circular with a typical radius of 3-7 .mu.m, which is sufficiently small to achieve high intensity over long interaction lengths with only modest optical powers of 50-100 mW. The light intensity required to allow significant amplification is quite high, typically 0.5-1.0 MW/cm.sup.2 for erbium-doped fiber amplifiers and 2.0-5.0 MW/cm.sup.2 for praseodymium-doped fiber amplifiers. With a coupling efficiency of 50%, this requires laser diodes with output power of 100-200 mW, and such laser diodes are commercially available further improvements in amplifier gain can be obtained by increasing the intensity of the pump light in the guided-mode region of the fiber. This can be achieved by reducing the radius of this region, but this increases the numerical aperture of the fiber which may have the effect of reducing coupling efficiency. Alternatively, increasing the output power of the laser diode will increase the intensity; However, the current state of the art does not yet allow the fabrication of reliable single spatial mode laser diodes with output power greatly exceeding 200 mW that have sufficient operating lifetimes to be suitable for optical fiber communication systems. In the case of praseodymium-doped fiber amplifiers, which require even more optical pumping power than erbium-doped fiber amplifiers, it is often essential in some amplifier configurations to couple power from more than one laser diode into the doped fiber. Consequently, combining the power from more than one laser diode is an attractive alternative for increasing the intensity of the optical pump power in dope fiber lasers and amplifiers.
It is known that means exist for combining and scaling the power from two optical sources with orthogonal polarizations or with different wavelengths. Devices fabricated with birefringent materials, such as a Wollaston prism, have been employed to physically separate an unpolarized or partially polarized light beam into separate beams of orthogonal polarization that propagate in different directions. Ideally, the sum of the optical power of the separated beams equals that of the incident beam provided there is no absorption of light in the beam splitting device. In its reverse configuration, however, such a device can be used to combine light from two polarized optical sources into a single beam of unpolarized light, thus combining the optical power. Devices that can perform such a function are numerous and are known to those skilled in the art. Such devices include those based on birefringent crystals, on dielectric coatings deposited on the interface between two glass prisms, and fiber optical devices that combine the light in two polarization-preserving fibers into a single fiber. In the case of different wavelengths, WDM multiplexors have been employed to combine and scale the power of two optical sources to provide a higher output on a single fiber. Such WDM devices may be comprised of dichoric mirrors, conical fused couplers or proximity couplers.
The characteristics of a rare earth doped fiber amplifier or laser is highly dependent on the characteristics of the pump laser. In particular, unwanted fluctuations or instabilities in the optical power or wavelength of the laser diode can cause corresponding fluctuations in the amplification of the signal in an amplifier or in the output power of a laser. Because the response time of the excited state of rare earth elements in glass is approximately 3.times.10.sup.-3 s to 10.sup.-2 s, pump laser instabilities on a shorter time scale are not manifested in the amplifier or laser operation. The most troublesome source of pump laser diode instabilities on this time scale is noise in the form of laser longitudinal mode hopping and wavelength and intensity fluctuations caused by unwanted optical feedback into the laser diode or changes in temperature or injection current. This noise is especially detrimental in rare earth doped fiber amplifiers because it increases errors in the amplified communication signal and detracts from the practicality of these devices.
Several techniques exist to reduce the effect of pump laser diode noise. One example is an active electrical system that detects the variation in output power of the fiber amplifier caused by a fluctuation in the laser diode characteristics and this signal is fed back into the pump laser at the correct phases to reduce the laser fluctuation. Unfortunately, this technique adds cost and complexity to the amplifier. For this reason, it is preferable to employ a passive method of reducing laser diode fluctuations. An attractive solution is to feed back into the pump laser diode a fraction of its own light. These laser diodes are very sensitive to optical feedback, and if such feedback is properly controlled, improved laser operation can result. Feedback is usually provided by an external reflector such as a mirror or diffraction grating, and external optical elements, such as lenses, are required to manipulate and guide the light out of and back into the laser diode cavity. Although the external optics and reflectors can often be quite compact, it is difficult and expensive to align such a system, and the mechanical and thermal stability can often be inadequate for use in fiber amplifiers and lasers.
It should now be appreciated that a method to couple more optical pump power into rare earth doped fiber amplifiers from semiconductor laser diodes is desirable. It is also necessary to provide a simple, convenient and mechanically rugged means for reducing the wavelength and intensity instabilities for the pump laser diodes.